Durability of Urea-Resin Glues At Elevated Temperatures

The performance of several glue types, including high and low solids, liquid and powdered resins, a fortified liquid resin, and a non-crazing resin were evaluated in continuous exposure at 80?F. and 30 or 65 percent R.H.; 120?F. and 28 percent R.H. 140?F. and 25 percent R.H.; and 158?F. and 20 percent R.H. Eleven glues were used with birch plywood and five with sweetgum plywood, with and without wheat flour extender, and cured both in hot presses and at room temperature. Four of these glues were used in hard maple block-shear test specimens to furnish strength data at a higher level. Shear strength of birch plywood from both curing methods, exposed at 80?F. and 30 percent R.H., showed little change in 36 months. After 36 months at 158?F., the average strength value of block-joints with all glues was only 52 percent of the average strength value of the same glues after 36 months at 80?F. The average age strength value of all the hot-press glues in both birch and gum plywood after 36 months at 158?F. was 43 percent of the average strength value after the same period of 80?F., and the corresponding figure for the room-temperature glues was about 30 percent. Urea-resin glues, regardless of type or extension, were weakened by continued exposure to temperatures from 120? to 158?F. The higher the temperature of exposure, the greater the effect. In general, the order of performance of the different urea resins at 120?F. or higher was the same as that of initial control strength levels. Increases in strength values often occurred during the first 6 months’ exposure to 80?F., and levels of performance over long periods of exposure to 80?F. were generally more directly related to these 6 months test values than to the initial controls. Hot-press glues generally had higher initial strength values and retained levels of greater strength after elevated temperature exposure than room-temperature-cured glues. In general, the high-solids, liquid urea-resin formulations were somewhat superior to formulations of powdered resins, and these in turn were better than formulations of low-solids, liquid resins. Wheat flour extender, as such, had no pronounced effects on heat resistance of any of these resins.

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